Thread: Bad Gas Mileage

Hello fellow Pontiac Grand Prix owners...I recently posted a thread about getting better mileage by fixing the exhaust leaks...well it lasted for about a week...

I mostly do city driving and my car does about 15mpg, which is really bad...

I recently changed the spark plugs and the fuel filter while doing a carburator cleanup and a vacuum cleanup...I resetted the pcm numerous times...

I never had a code thrown in concerning the 02 sensors or a bad catalyctic...

Is it possible my overkill pcm won't throw the codes?

Please help, what are my options to increase mileage? Or is it simply normal considering I always do city driving and let's not forget it's winter over here in Canada.

Thanks for your precious advice!

Well, I’d say Will probably would have disabled the cat code. That being said, I seriously doubt your cat all of a sudden plugged up. And if it did you would have very noticeable drivability issues. Considering your car is only rated for about 16-17 mpg city by the EPA, 15 mpg isn’t unforeseeable by any measure. How many miles do you have racked up? If you at or near 120k I’d be looking for a new front O2 sensor. It is frighteningly predictable how active mpg topics become around Dec, Jan and Feb. Then the rest of the year hardly a peep about mpg, just something to consider.

Originally Posted by cerick08GP

Well, I’d say Will probably would have disabled the cat code. That being said, I seriously doubt your cat all of a sudden plugged up. And if it did you would have very noticeable drivability issues. Considering your car is only rated for about 16-17 mpg city by the EPA, 15 mpg isn’t unforeseeable by any measure. How many miles do you have racked up? If you at or near 120k I’d be looking for a new front O2 sensor. It is frighteningly predictable how active mpg topics become around Dec, Jan and Feb. Then the rest of the year hardly a peep about mpg, just something to consider.

Well then, if the car is rated for about 16-17 mpg then I think there is nothing to worry about?...My car has about 60k miles...

Now that I know you have only 60k on your car, I’m almost positive your fine. And just for kicks I did look up your EPA city, it is 16 mpg. If you ever start to really track your mpg, you and everyone else dealing with cold winters will notice a pretty big hit in mpg. It is not abnormal for ppl to see a 10-15% drop in mpg. However it is very common for most GP owners to beat the EPA estimates on average, but the mpg ppl get in real life is affected by a few dozen factors. I can give you one little tip for the future that most ppl don’t consider, and that is to keep track of your DIC average speed along with your average mpg. Those two numbers are pretty much attached at the hip, the lower you average speed the lower the mpg. It sounds counter intuitive, but in practice it works. At least for the majority of us (myself included) that drive on city roads and highways most of the time. For the very few “I drive on the interstate 80%+ of the time” I can’t a test to that. In my experience, if you average about 35mph or higher you’re going to meet or beat EPA mpg. At or below 34mph you’re looking at EPA mpg or lower. (just rough numbers) And just to clarify, I’m talking the average speed over a fill up not instantaneous readings. Try it out

Thanks for your complete answer! Actually, my average speed is quite low...actually terrible...around 20mph

It does make sense, the lower your speed, the less mpg you get. When you check out your screen, mpg goes up as you speed up...

I'm going to check it out again this summer and reset the numbers just before a road trip. Gotta though it out this winter

Thanks again!

Glad I could set your mind at ease Ace. Hehe Although it is probably showing, fuel efficiency is kind of a hobby of mine. (Ya ya I know )

mlasnoski, you are absolutely correct. The car uses a combination of the speedometer and the engine hour gauge to take into account idle time in the average speed calculation, this is a very useful tool. Although a person could come to really messed up conclusions using the instantaneous DIC readings (they tend to all over the place), however over the course of a couple fill ups you can get some really good data.

As I mentioned earlier the faster average speed you have, your mpg will track up with it. In my experience you hit a wall at about 50 mph average speed, then mpg will start to track down with speed increase. This is of course due to aerodynamic loses at higher speeds.

The truth is GPs are awesome highway/interstate cruisers, and they get great mpg under those conditions. But they really take a big hit in stoplight to stoplight city driving, as due most midsized vehicles.

Originally Posted by cerick08GP

Although a person could come to really messed up conclusions using the instantaneous DIC readings (they tend to all over the place)

When you say instantaneous readings am I missing something? I didn't think there was a way to check "at the moment" mpg readings. I only have an option to see averaged readings on my DIC. Is there a trick to seeing instant readings?

Yes, my GP has an instantaneous mpg read out. However, as I mentioned earlier IMHO it isn’t very useful as it pounces around quite a bit even when the car is operating at/near steady state.
I’m pretty sure all GPs with the 5-button DICs have this feature, but I’m not sure when they added it. Just a guess but it was probably around 05-06 model years. If you do have it, you can display it by pushing the button with the gas pump symbol on it and cycle through the options until it reads “INST ECONOMY”.

Originally Posted by mlasnoski

One time before I put new tires on; I averaged 33mpg. Now its between 26-30 depending on wind and temperature. On my way back up North from Milwaukee, WI I only averaged about 25, but I think the cold weather and the head on wind decreased that. =)

I’m having a B-M moment with that comment. Just last week I was reading a study done by the state of California on the rolling resistance of tires. And your observation is pretty spot on with their testing. As a general rule of thumb, as a tire wears the rolling resistance decreases yielding slightly increased mpg. However, I’m talking about a ~1mpg difference max. But throw in the fact you most likely bought non-OE tires, I can definitely see you losing 1-2 mpg. OE tires are crappy and hard as #eLL put they have a pretty decent fiction coefficient for mpg.

I'm not 100% sure on this, but I was told by someone at a refinery that in ND the gas is not produced identically in the winter as the summer. I wasn't following the whole discussion, however the idea was the flashpoint is (I believe) lower in the winter due to different additives that will also decrease fuel mileage. Like other said, cold weather never helps either as our cars run crazy rich when they start-up cold. I actually tuned mine to not run as rich for as long in cold weather. Snow also increases roll resist

I get 16-17mph in the middle of winter (and I don't leave my car run much except for about a minute when it is below 0 just to not be as hard on the motor). In summer I get 18-20 depending almost entirely on my right foot.

Originally Posted by lightfoot__

I'm not 100% sure on this, but I was told by someone at a refinery that in ND the gas is not produced identically in the winter as the summer. I wasn't following the whole discussion, however the idea was the flashpoint is (I believe) lower in the winter due to different additives that will also decrease fuel mileage. Like other said, cold weather never helps either as our cars run crazy rich when they start-up cold.

All true

And just to specify on the gas, the common term is “winter blend” gasoline. And it does have somewhere between 2-3% (depending on who gives the number) less energy content than the summer version.